Would it be ethical to use AI to determine a user’s gender from the content they upload, without them knowing?
Ethics aside, I think this could potentially create a number of different issues. What if your algorithm guesses wrong and I'm stuck with a UI that's targeting incorrectly? What if my wife and I share an account? What if I'm a (insert orientation here) (insert gender here) who [coaches, supports] a [men's, women's] [volleyball, football] team? People are very diverse, and you could make many mistakes. Would you then provide tools for the user to correct these mistakes? How would you be able to do so without the user being offended?

Instead, I see many fewer issues that might arise from a section like the following:


  Let's get to know each other!
  
  If you'd like, we can help tune your profile to match your interests.
  
  You can start by selecting some of your interests from the list below, or try searching for your own.
  
  Fishing Cooking Soccer Video Gaming Social Media
  Search for my interest...
  
  Skip this section &rarr;


You could continue with other questions that might actually be relevant to your shaping your UI or application, allowing the user to omit/delete details for any or all questions.

Being purely an opt-in experience prevents any issues with the user feeling like your app may be "talking about them" behind their back.

In this case, specifically at this point in time, I think it's wise to be transparent about what data your application knows (or thinks it knows) about your users.